The present invention concerns formation of thin crystalline films of compounds and deals more specifically with an apparatus for depositing high purity films of compounds such as zinc sulfide using desired substrates on a concept of what is termed "hot wall epitaxy".
Demands for thin films of compounds are ever increasing with the development of microelectronics and optical communication technology. Such films find use in a variety of devices for the conversion of electrical energy into light energy (e.g. semiconductor lasers, light emitting diodes, and electroluminescent lamps) and those for the conversion of light energy into electrical energy (e.g. infrared sensors and solar cells). All these devices require films of the highest purity and crystalline perfection. The addition of impurities to the films must also be well controlled.
We are aware of a great variety of methods heretofore suggested and practiced for the formation of crystalline compound films. The molecular beam method is perhaps the only one of such known methods that is capable of epitaxially growing the crystalline films of a diversity of compounds. This advantage is offset, however, by drawbacks such as (a) an extremely high degree of vacuum required, (b) bulky and expensive equipment, (c) considerable waste of the source materials, and (d) difficulties in the processing of large size substrates.
Liquid phase epitaxial growth is another well known method. The high temperatures required by this method, however, make it unapplicable to materials that develop high vapor pressures. It also involves difficulties in the processing of large size substrates. There also are various vapor phase methods of epitaxy, such as chemical vapor deposition, ALE, cathode sputtering, and vacuum evaporation. The disadvantages of these vapor phase methods are (a) high substrate temperaures, (b) limitations in the source materials that can be employed and in their possible combinations, and (c) the tendency of the deposited films to become amorphous.
Thus the prior art methods have all one drawback or another. We further wish to invite attention to the fact that molecular beam epitaxy has so far been probably the sole method capable of producing zinc sulfide films with high quality. As far as we are aware, the successful formation of zinc sulfide films by other methods has not been reported.